A common approach to environmental control system design for vehicles using turbo machines is based on the air cycle machine or "pack". The pack outputs cool air at a pressure sufficient to move the air through distribution ducts and at a rate sufficient to satisfy the fresh air needs of the occupants (while also compensating for intentional and unintentional outward leaks). The air cycle machine is said to emulate the "air-standard refrigeration cycle" found in thermodynamic texts. However, differences exist which provide the opportunity for increasing efficiency of the traditional air cycle machine approach.
The typical air cycle machine extracts an amount of "bleed air" from the engine core compressor ports, such as the eighth stage (low stage bleed air) or fourteenth stage (high stage bleed air). The bleed air is compressed or pressurized and as a result heated to a very high temperature relative to ambient. The bleed air is then passed through a compressor which further increases the temperature and pressure. Ram air flow is used to cool the compressed bleed air before it is expanded and further cooled in a turbine to an appropriate temperature and pressure to operate a water separateor and cool the controlled space, generally referred to herein as the cabin.
In terms of engine fuel consumption, bleed air is very expensive. For example, in a typical aircraft application cruising at 35,000 feet, one pound per second (pps) of low stage bleed air costs the same as 158 kilowatts (211 hp) of shaft power extracted from a gearbox coupled to the engine: 1.2% specific fuel consumption (SFC). Furthermore, fan air used to precool the bleed air before passing it to the compressor may add an additional 0.52% SFC. As such, it is desirable to reduce or eliminate the use of bleed air to increase efficiency of the environmental control system. Alternatively, it is desirable to fully utilize the energy of the bleed air.
For aircraft applications, cabin or fuselage pressure is regulated by restricting outflow of air through an outflow valve. The outflow valve has an open area modulated to provide a desired pressure. To maintain a constant cabin pressure level, the rate of air supplied must equal the rate of air leaked plus the rate of air exhausted through the outflow valve. In an attempt to recover energy from the outflow air by converting it to forward thrust, the outflow valve is carefully shaped to form a converging-diverging supersonic nozzle since the pressure ratio allows an exit Mach number of about 1.6 in a single process. The actual savings in % SFC are difficult to isolate and measure due to various factors, such as the possibility of creating an adverse yaw moment which requires rudder compensation thereby leading to additional aerodynamic drag. As such, it is desirable to provide an alternative approach to energy recovery to improve the efficiency of the environmental control system.